The present invention relates to pumps in general, and more particularly to improvements in slurry feed pumps, especially for use in coal liquefaction reactors. Such pumps are utilized to convey, at elevated pressures and temperatures, a suspension consisting of coal particles, a catalyst and an oil, especially coal oil.
Slurry feed pumps are used in coal conversion plants to supply a suspension of fragmentized coal, coal oil and a catalyst to a high-pressure reactor or gasifier for direct hydrogenation of hard pit or brown coal, i.e., for conversion of coal into a liquid hydrocarbon. The function of slurry pumps in such plants is to convey the suspension into a hydrogenating reactor at a temperature of between 300.degree. and 450.degree. C. and at a pressure of between 200 and 300 bar. The solids content of the suspension is as high as 50 percent by weight, and the hard fragments of solid material in the suspension cause a considerable amount of abrasion. Therefore, erosion of the pump components is highly pronounced. Moreover, the suspension often contains corrosive substances which, at the aforementioned elevated temperatures and pressures, contribute to rapid wear upon the parts of such slurry feed pumps. Attempts to avoid excessive and premature wear upon slurry feed pumps include the utilization of indirect conveying systems, for example, systems which operate with piston type pumps. A drawback of such systems is excessive initial and maintenance cost as well as the absence of delivery of suspension at a constant rate. Moreover, presently known large-scale hydrogenating plants are invariably designed for multiple division of suspension into a plurality of smaller streams, i.e., resort is made to a parallel or concurrent type operation.
Problems which arise in connection with extensive wear upon high-pressure centrifugal slurry pumps are discussed in the article entitled "Development of a High-Pressure Centrifugal Slurry Pump" by G. S. Wong et al. appearing on pages 58 through 65 of the December 1979 edition of CEP. The writers of the article point out that the parts most affected by erosion under the action of coal-oil slurries are the volute cutwater, the impeller wear ring, the leading edges of impeller wanes, the primary seal, the volute casing (especially its suction side) and the volute collector. Other literature dealing with presently known slurry feed pumps for use in coal liquefaction reactors includes the article entitled "Coal Slurry Feed Pump for Coal Liquefaction" by G. S. Wong et al. (prepared in September 1978 for Electric Power Research Institute, EPRI AF-853, Project 775-1, Final Report), and "Survey of Industrial Coal Conversion Equipment Capabilities: Rotating Components" by W. R. Williams et al. (April 1978, Oak Ridge National Laboratory, ORNL/TM-6074). Certain U.S. Letters Patent are believed to have been granted for slurry feed pumps to the Electric Power Research Institute of Palo Alto, Calif.